Jennifer Mazzotta-Perretti never expected that, after giving students an assignment to write about their experiences doing good deeds, she would have the opportunity to do one herself.

One student in her summer creative writing class at Nassau BOCES in Wantagh posed the question: Would you give life to someone else if you didn't have to give up your own? She said yes, she would.

Then he asked if she would donate one of her kidneys - to him. Again, she said yes.

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At the time the student, Kevin O'Brien, didn't need a transplant. Later, when he did, he remembered her answer and asked her again.

She pledged that she would, not expecting it to work out because the odds were against two unrelated people being a match.

But after a blood donor card arrived in the mail stating that her blood type was O positive - the same as his - she felt compelled to undergo more testing and learned that their match went beyond blood.

That's when the single mother of one from Levittown prayed - and decided to go through with the donation.

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"It was an awesome feeling that I was going to help this kid with more than reading and writing," said Mazzotta-Perretti, 32, who is also the special education director at the Hebrew Academy of the Five Towns & Rockaway.

On Sept. 3 at Columbia University Medical Center, Mazzotta-Perretti fulfilled her promise and gave 19-year-old O'Brien one of her kidneys.

O'Brien said that after years of feeling tired, he immediately felt energized.

"You wake up and you're like, 'Whoa, is this for real?' " said O'Brien, of Oyster Bay. "I feel better than I have in quite some time."

When he was 3, blood drawn from a finger prick led to the discovery of an obstruction in one of his ureters - tubes that connect the kidneys to the bladder - that was causing urine to back up and damage the right kidney.

Within months, surgeons implanted the tube deeper into the kidney to prevent the reflux. Two years later, the same procedure was necessary for his left kidney. But the surgeries were a temporary fix: Doctors said he would eventually need a kidney transplant.

A decade later, on July 1, 2005, O'Brien received a kidney from his father, Neil. But soon after, that kidney began to fail because the drugs he was taking to keep his body from rejecting the new kidney made him susceptible to a virus, which damaged the kidney.

His mother, Heidi, wasn't a match for her son and tried to arrange a kidney swap: She would donate a kidney to someone she did match - and who, in turn, would provide a willing donor who was a match for Kevin.

That strategy didn't pay off, and Kevin's name was put on a waiting list - where it could have taken him eight years to get to the top.

"We were devastated," said Heidi O'Brien, 52, of Oyster Bay. "We had done everything we could do."

Then Mazzotta-Perretti called her and said she would give Kevin her kidney. Heidi O'Brien said she was "in awe that a person would do that for my child."

"We are so grateful to Jennifer," she said.

To pay Mazzotta-Perretti's favor forward, Kevin O'Brien said he wants to work with scientists to clone human organs. He said he also wants to encourage people to donate the organs of their deceased loved ones and pledge to donate their own organs when they die.

"Give the organs to someone who can use them," he said.

But first, now that his health is improving, O'Brien will have to finish his junior and senior years of high school.

Looking back on his quest for a donor, O'Brien said the best thing he did was put his teacher on the spot.

"You gotta not be afraid to ask for what you need," he said. "I needed a kidney and I have it."